1. Field
The invention relates to a bone anchoring device for anchoring a stabilization rod in a bone or in a vertebra. The bone anchoring device includes an anchoring element, a receiving part for receiving a head of the bone anchoring element and for receiving the stabilization rod to be connected to the anchoring element. The anchoring element is pivotably connected to the receiving part and can be fixed at an angle by exerting pressure onto the head via a pressure element which is arranged in the receiving part. The pressure element has a flexible portion to clamp the head. The pressure element also has a securing portion which engages a securing portion at the receiving part to prevent a loss of the pressure element after insertion of the pressure element into the receiving part.
2. Description of Related Art
U.S. Pat. No. 5,716,356 describes a polyaxial bone screw including a screw element and a receiving part which is pivotably connected to the screw element and a pressure element to exert pressure onto the head of the screw element to fix the angle between the screw element and the receiving part. The pressure element is held in the receiving part in an aligned position by means of crimping through bores provided in the receiving part.
U.S. Pat. No. 5,672,176 describes another example of a polyaxial bone screw with a pressure element which is also held in place by crimp bores without impeding a sufficient movement of the pressure element to clamp the head.
WO2006/116437 A2 describes a bone anchor for spinal fixation in the form of a polyaxial bone screw including a screw element, a housing, a sleeve and a collet arranged in the housing for exerting pressure onto the head of the screw element. The sleeve has retention tabs which snap into slots in opposite wall positions of the housing.
The polyaxial bone screw described in DE 43 07 576 C1 has an inner thread at the receiving part which engages an outer thread of the pressure element as well as an outer thread of a clamping screw which clamps the head. WO98/34554 describes a multi-axial bone engaging fastener assembly wherein a crown member presses onto the head of the bone screw. The crown member is threaded and inserted by threading it through the threaded portion of the receiver member. Therefore, it cannot escape inadvertently.
Usually, the polyaxial bone anchoring devices of the above described types are provided, for example, by the manufacturer, in a pre-assembled condition. In this condition a specific screw element, for example, a screw element having a specific length and a specific shaft diameter or specific thread form is connected to the receiving part, and the pressure element is arranged therein so that it cannot fall out. For surgery, the necessary number and types of such pre-assembled polyaxial bone screws are selected and provided in advance as a full set of implants.